This application claims the priority of German application 198 37 178.0, filed in Germany on Aug. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an arrangement in a ring spinning machine comprising a front roller pair which ends the main drafting zone of a drafting assembly and also comprising a condensing zone downstream thereof, which condensing zone bundles a drafted fiber strand and which condensing zone is ended by a driven nipping roller which presses the fiber strand against a supporting surface, the peripheral speed of said nipping roller being adapted to the peripheral speed of the front roller pair.
An arrangement of this type is described in U.S. Pat. No. 5,600,872, in which the peripheral speed of the nipping roller and the front roller pair are calculated in such a way that the necessary fiber strand tension for a fault-free transport is ensured. Excessive tension is disclosed there as being undesirable as this would impair the bundling of the fibers.
It has been shown that in the case of such a measure, the optimal tear strength of the spun yarn is not yet attainable. It is therefore an object of the present invention to measure the tension draft so that the tear strength of the yarn is increased.
This object has been achieved in accordance with the present invention in that the peripheral speed of the nipping roller is 3% to 8% greater than the peripheral speed of the front roller pair.
Such a ratio of the peripheral speeds serves not only to provide the necessary tension to ensure a fault-free transport of the fiber strand, but also leads furthermore to a lasting, if slight, draft. The condensing zone thus becomes intentionally subject to draft according to the present invention. The yarn tear strength is hereby significantly increased. The view of experts in the textile industry that a fiber strand could only be condensed in an almost completely draft free zone could not be confirmed.
With regard to the drive, the nipping roller is advantageously connected to a roller of the front roller pair. This has the advantage that, in the case of a change in the drafting drive, the drive of the nipping roller is automatically altered by the correct amount.
In an embodiment of the present invention a sieve belt is arranged at the nipping roller, which sieve belt transports the fiber strand with the peripheral speed of the nipping roller over a suction slit. The suction slit is located for this purpose in a stationary sliding surface. The sieve belt has two functions, the first being namely to guide the fiber strand through the subsequent drafting zone and the second being to homogenize an air stream for pneumatic condensing. The sieve belt can be driven by a nipping roller, or in reverse, the sieve belt can drive the nipping roller.
It is advantageous when the sieve belt is smoother on its side facing the sliding surface than on the side facing away from the sliding surface. This takes into consideration the fact that the sieve belt on the one hand slides on a stationary sliding surface, and on the other hand is a component part of a friction drive. In order that the fiber strand is disposed closely fitting on the sliding surface, said sliding surface is curved in transport direction for this purpose.
For ease of operation, it is practical when the nipping roller is integrated into the top weighting arm already present in the relevant drafting assembly.